1. Field of the Invention
The invention concerns a dishwasher pump drive which comprises an operating circuit without a rotary speed control for the pump motor thereof.
2. Discussion of the Prior Art
It is typical practice to fit to a dishwasher a single-phase asynchronous motor as the drive for its circulating pump, which for the purposes of producing a rotating field by way of a capacitor generates a voltage variation in a second phase position (see EP 1 075 080 A2) and whose rotary speed fluctuates in dependence on load with the consequence of varying noise emission. From the structural point of view such a drive suffers from the disadvantage of requiring a seal between the motor and the pump, which is there subjected to a frictional loading, and for that reason it is susceptible to wear and is therefore a source of additional interference noises which are not constant in terms of spectrum and intensity.
A structurally more advantageous pump drive would be a synchronous motor; more specifically, by virtue of its wide permissible air gap between the stator and the rotor, into which it would be possible to fit a plastic cup which is open towards the water side and in which the rotor would then be supported in the form of a wet rotor member. That would avoid sealing arrangements between the motor and the pump, which are susceptible to wear and which are detrimental in regard to noise. However such a motor cannot start automatically by virtue of its mass moment of inertia, more particularly under load, so it requires an expensive electronic acceleration control to go into a mains-synchronous operating mode. In addition, for reliable starting, an asymmetry of the field is required, more specifically an air gap which is not constant in the peripheral direction. That results in a torque which fluctuates in the rotational movement and which in operation results in an unacceptable noise nuisance. For that reason that drive has not acquired any significance on the market.
A desirable arrangement for start-up control for gentle and accordingly low-noise operation or for optimising the speed for different, energy-optimised washing programs would be possible with multi-phase symmetrical synchronous motors which however also require expensive and complicated electronic control circuits for operation thereof, the use of such circuits being precluded in dishwashing machines for cost reasons.
In order to be able to meet cost constraints which are predetermined by the competition situation in the market, it is therefore necessary in practice to be satisfied with unregulated asynchronous motors as the drives in dishwashers, and it is then necessary to accept the generation of noise, as typically occurs for example when switching on the pump motor, by virtue of the steam surge or jet when changing between a hot water treatment and a cold water treatment, or after pumping out the water when the pump runs empty.
The object of the present invention is to provide as a substitute for the inexpensive asynchronous drive a dishwasher pump drive which enjoys the structural advantages of a wet rotor member and which if necessary also permits inexpensive rotary speed control for the purposes of optimising operation of the motor from an electrical and acoustic point of view; and which therefore in spite of the large air gap between the stator and the rotor can be operated with a high level of efficiency and a low level of noise over a wide range of rotary speeds without having to lose out on the previous costing aspect, due to the more complicated and expensive circuitry which is required for that purpose.
In accordance with the invention that object is attained by the combination of the essential features recited in the main claim. In accordance therewith, the pump drive is a dc motor with a permanent-magnetic rotor which can still be operated with a good level of efficiency even with a comparatively large air gap and which permits sensor-controlled electronic computation with particularly simple switching means. In addition it is now possible for additional rotary speed control to actually have access to the operating circuit for a dishwasher, which is already present as it is used as standard and which hitherto does not yet permit rotary speed control of the pump motor.
In accordance with a development of the invention, for that mode of operation, the motor housing is equipped with an additional circuit for connection to the conventional operating circuit. A pulse sequence which is of high frequency in relation to the repetition rate of the commutator signals and which can be derived from the processor of the operating circuit and which has a variable frequency or a pulse duty factor which is now variable switches the field current circuit without the need to have to provide for that purpose the more expensive and complicated conventional half-bridge circuits, and thus by way of simple change-over switches, in the manner of a chopper control arrangement (principle of the reducing adjusting device), provides a variable mean motor voltage for influencing the rotary speed of the pump motor.
Still more advantageous than direct rotary speed control is indirectly influencing the rotary speed by way of the mean motor current. For, regulation to a constant motor current in accordance with that development of the structure according to the invention directly involves influencing the torque and thus by way of a constant torque results in particularly smooth operation of the electronically commutating dc motor.